As is generally known in the art, a continuously variable transmission comprises a planetary gear unit composed of a ring gear having a predetermined degree of freedom, a planetary gear, a carrier and a sun gear, and a transmission, which are combined with each other in order to construct a belt type continuously variable transmission, so that fine speed adjustment is made possible through continuous shifting, and a wide range of shifting ratios can be obtained.
A belt type continuously variable transmission combined with a planetary gear unit is constructed to transmit power through friction. Therefore, in order to obtain a required level of friction, a substantial amount of power loss is caused due to operation loads and slippage of component parts. Also, since the belt type continuously variable transmission has a complicated structure and low efficiency, it cannot be used in a bicycle or a small power device, and can only be limitedly used in heavy equipment or an industrial machine to which a heavy load is applied.
In the belt type continuously variable transmission, a drive belt is wound on a pair of variable pulleys so that shifting can be implemented by changing the diameters of the pulleys. In order to obtain a high level of speed reduction, the sizes of the pulleys must be relatively increased, and therefore, the size of the entire transmission is enlarged. Also, since the contact area between the belt and the pulleys is increased, operational loads are increased, and it is difficult to apply the belt type continuously variable transmission to intermediate and small power devices.
Meanwhile, in the case of a bicycle, a manual transmission is adopted, in which two or three sprockets are installed on a pedal, and seven or eight sprockets are installed on a rear wheel, such that a chain can be moved through the manipulation of operation levers to effect a speed change. However, in this type of transmission, power transmission is interrupted at the time that the chain is moved through manipulation of the operation levers, and the construction of the transmission is complicated. Also, since two operation levers are used for shifting, usage is inconvenient, and a predetermined degree of skill is required.
Further, in the case of a motorcycle, a belt type continuously variable transmission using centrifugal force produced by engine RPM is adopted. Nevertheless, in this belt type continuously variable transmission, since operation loads of component parts and power loss due to slippage of pulleys are increased, engine RPM is unnecessarily raised, thus decreasing fuel economy and reducing shifting efficiency, and usage of the continuously variable transmission is limited for an engine having a low displacement.
In order to solve the above-described problems, the present applicant disclosed a power classification type continuously variable transmission in Korean Patent No. 436744.
The power classification type continuously variable transmission disclosed in Korean Patent No. 436744 comprises a simple combination of a planetary gear unit composed of a carrier, a ring gear having a predetermined degree of freedom and a sun gear, and a second transmission composed of an oscillation type actuator, that is, cam mechanisms and links, so that a high speed reduction ratio can be conveniently realized, and output and input levels can be adjusted as desired.
Nonetheless, because the power classification type continuously variable transmission has a relatively complicated construction, when it is applied to a simple device and machine, power loss results due to noise or vibration generated while driving force is transmitted from an input section to an output section. Also, the power classification type continuously variable transmission makes it difficult to minimize the weight of a device which is required to be light.